[unreadable] The precise regulation of neural excitability is essential for proper nerve cell, neural circuit and nervous system function. An increasing body of experimental evidence supports the conclusion that homeostatic signaling mechanisms can modulate synaptic function and contribute to the maintenance of stable neural activity (Marder et al., 1996; Davis and Goodman, 1998; Turrigiano and Nelson, 2000; Davis and Bezprozvanny, 2001; Marder and Prinz, 2003; Turrigiano and Nelson, 2004). We have previously demonstrated the presence of a homeostatic signaling system at the Drosophila NMJ that may require a retrograde signal from muscle to nerve (Davis and Bezprozvanny, 2001). In the human disease myasthenia gravis, and in mouse models of this disease, a similar homeostatic regulation of presynaptic transmitter release is observed (Plompp et al., 1992; Sandrock et al., 1992). To identify the molecular mechanisms of homeostatic signaling we have performed a screen of 1000 gene-specific dsRNA. We identified the Drosophila homologue of the Rho-GEF Ephexin as a gene that, when knocked down, blocks synaptic homeostasis. We have gone on to identify an ephexin loss-of-function mutation, and have confirmed that an ephexin mutation also blocks synaptic homeostasis. In mammalian neurons Ephexin binds to the EphA4 receptor (Shamah et al., 2001). We now show that the Drosophila Eph receptor is a synaptic protein that localizes to the NMJ and we show that dsRNA to the Eph gene also impairs synaptic homeostasis. This proposal is focused on characterizing the role of the Drosophila Eph receptor and ephexin genes during synapse development and synaptic homeostasis. We will address the exciting possibility that the Eph and ephexin genes define the basis of a synaptic signaling cascade that is required for synaptic homeostasis. Although the majority of our experiments focus on Eph-Ephexin signaling, we will also test the involvement of the Ephrin ligand during synaptic homeostasis. We will test whether Ephrin-Eph signaling mediates the retrograde signal that is believed to be involved in synaptic homeostasis at the NMJ. [unreadable] [unreadable]